Protective layers for optical coatings

ABSTRACT

An optical coating on a transparent substrate is provided with a temporary layer of carbon as protection during manufacturing against scratches and corrosive environments. When the optical coating and/or substrate are tempered in an atmosphere reactive to carbon, such as air, the layer of carbon is removed as a carbon-containing gas. For an optical coating with a brittle, glassy, outermost layer furthest from the substrate, additional protection is provided by a scratch propagation blocker layer between the outermost layer and the carbon protective layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to protective layers applied to optical coatingson transparent substrates. In particular, the invention relates to theuse of a temporary protective layer of carbon. In addition, theinvention relates to a scratch propagation blocker (SPB) protectivelayer applied to the outermost layer of various optical coatings.

2. Discussion of the Background

Optical coatings are deposited on transparent substrates to reflect orotherwise alter the transmission of some or all of the radiationincident on the substrates. For example, the optical coating of a mirroris designed to reflect visible light. Low-emissivity optical coatingsare designed to reduce the transmission of infrared radiation. Opticalcoatings generally include two or more different layers each having athickness in a range of from less than 1 nm to over 500 nm.

Optical coatings are frequently damaged during shipping and handling byscratching and by exposure to corrosive environments. Silver basedlow-emissivity coatings in particular have been plagued with corrosionproblems since their introduction into the fenestration marketplacedecades ago. Attempts at improving the durability of optical coatingshave included the application of a temporary protective layer such as aplastic adhesive backed film. Other protective layers have been formedby applying and curing solvent based polymers on glass.

However, a number of problems are associated with using adhesive filmsand polymer films as protective layers on optical coatings. Expensive,specialized equipment is required to apply the adhesive films and thepolymer films to optical coatings. When an adhesive film is pulled awayfrom an optical coating, the adhesive film runs the risk of removingportions of the optical coating. Even if portions of the optical coatingare not removed, the force on the optical coating associated withremoving the adhesive film can damage the optical coating. A solventbased polymer film applied to an optical coating must be dried and thesolvent removed in an environmentally friendly manner. Removal of thepolymer film from an optical coating requires specialized washing thatcan easily damage the optical coating.

For protection from corrosion, most silver based low-emissivity stacksin use today make use of barrier or cladding layers in direct contactand on one or both sides of the silver layers. It is well known in theart that various thin film layers can function as barriers to movementof corrosive fluids such as water vapor and oxygen. Metals layers areknown to be particularly effective diffusion barriers due to theirability to physically and chemically inhibit diffusion of corrosivefluids. Metal layers tend to be more effective physical barriers todiffusion than dielectric layers such as oxides, because both evaporatedand sputtered metal layers tend to contain fewer pinhole defects thanoxide layers. Metal layers also tend to chemically block diffusion byreacting with fluids diffusing through a pinhole to stop the movement ofall chemically bound fluid molecules. The bound fluid molecules in turnrestrict the passage of additional fluid through the pinhole. The morereactive metals are particularly effective for chemically blocking.

Tempering greatly reduces the corrosion problems associated with silverbased low-emissivity coatings. Tempering results in an atomic levelrestructuring to a lower energy state and renders the silver far lessprone to corrosion. Tempering also improves the hardness and scratchresistance of optical coatings.

However, until optical coatings are tempered, the coatings remainparticularly susceptible to damage from scratching and corrosion. Evenafter tempering, optical coatings are not immune from scratching andcorrosion.

Scratches in an optical coating frequently do not become visible untilafter the coating is heated and tempered, which can cause the scratchesto grow and propagate.

Carbon has been used as a protective coating on glass substrates. Forexample, U.S. Pat. No. 6,303,226 discloses the use of an amorphous,diamond-like carbon (DLC), protective layer on a glass substrate.

There is a need for improved methods and layers for protecting opticalcoatings.

SUMMARY OF THE INVENTION

The present invention provides a method of making a transparent articlewith a reduced number of scratches and other surface defects. Thetransparent article includes a optical coating on a transparentsubstrate. According to the invention, a protective coating is formed onthe optical coating that improves the durability and scratch resistanceof the optical coating, particularly during manufacturing.

The protective coating can include a layer consisting essentially ofcarbon. The carbon protective layer is formed on the optical coatingbefore tempering. During shipping, and handling of the untemperedoptical coating, the carbon layer serves as a low friction, protectivelayer against scratches. Heating and tempering the optical coatingand/or transparent substrate in an atmosphere reactive to carbonconsumes the carbon protective layer, thus eliminating any scratches orother surface defects in the carbon. The carbon protective layer isconverted into a carbon containing gas, leaving behind a relativelyscratch-free optical coating.

The protective coating can also include a thin protective layer of ascratch propagation blocker (SPB) material. The SPB material inhibitsthe propagation of scratches into the brittle, glassy, outermost layerof various optical coatings during tempering. SPB materials such as Ti,Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W, and oxides andnitrides thereof, are suitable for use on an outermost layer of siliconnitride (e.g., Si₃N₄). The SPB layer can be formed by depositing on theoutermost layer of an optical coating a diffusion barrier layer of atleast one metal, metal sub-oxide or metal sub-nitride of Ti, Si, Zn, Sn,In, Zr, Al, Cr, Nb, Mo, Hf, Ta or W; and then reacting the diffusionbarrier layer with an oxygen containing atmosphere such as air to form ametal oxide SPB layer including at least one of TiO₂, SiO₂, ZnO, SnO₂,In₂O₃, ZrO₂, Al₂O₃, Cr₂O₃, Nb₂O₅, MoO₃, HfO₂, Ta₂O₅ and WO₃. The SPBlayer can be used with or without a carbon protective layer on the SPBlayer.

Use of the temporary carbon protective layer when manufacturing atransparent article having an optical coating significantly reduces thenumber and severity of scratches introduced into the optical coating bythe manufacturing process. Because the carbon layer is removed duringtempering, the carbon layer does not affect the optical properties ofthe optical coating. While the SPB layer is not removed during temperingand may affect the optical properties of an optical coating, the SPBlayer, by inhibiting scratch propagation, is particularly useful inprotecting a brittle, glassy, outermost layer of an optical coating fromthe formation of visible scratches. A metal, metal sub-oxide or metalsub-nitride layer is particularly useful in providing corrosionprotection before tempering and can be converted by tempering in anatmosphere containing oxygen to a metal oxide SPB layer that isessentially transparent to visible light.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of this invention will be described in detailwith reference to the following figures.

FIGS. 1A-1C show the deposition of a carbon protective layer on anoptical coating on a glass substrate and the subsequent removal of thecarbon protective layer.

FIG. 2 shows a glass substrate coated with an optical coating, a scratchpropagation blocker layer and a carbon protective layer.

FIG. 3. shows the propagation of a scratch through a layer of Si₃N₄.

FIGS. 4A-4C show the deposition of a metal layer on an optical coatingon a glass substrate and the subsequent conversion of the metal layer toa metal oxide scratch propagation blocker layer.

FIG. 5 compares glass substrates, having the same optical coating butwith and without a carbon protective layer, when scratched.

FIG. 6 compares glass substrates, having the same optical coating butwith and without a carbon protective layer, when scratched.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a protective coating on an opticalcoating deposited on a transparent substrate to inhibit the formation ofscratches on and corrosion of the optical coating.

The transparent substrate can be a plastic or a glass. Preferably, thetransparent substrate is a glass that can be tempered by heating andquenching.

In embodiments, the protective coating includes a carbon protectivelayer. Carbon is a classic low-friction material. Even if an abrasivesucceeds in initially scratching carbon, the abrasive often becomescoated with carbon. Subsequent contact between the carbon coatedabrasive and carbon is characterized by one of the lowest coefficientsof friction, μ_(static)≈μ_(kinetic)=0.1 to 0.2. Thus, the carbon coatedabrasive tends to slide off of the carbon, doing no further damage tothe carbon. Carbon is also inert in many corrosive environments andexhibits good resistance to alkalies and most acids. Thus, a carbonlayer on an optical coating can protect the optical coating fromscratches and environmental corrosion during handling.

FIGS. 1A-1C illustrate embodiments of the invention in which a temporarycarbon layer is formed on an optical coating to protect the opticalcoating from scratching and environmental corrosion duringmanufacturing. FIG. 1A shows a glass substrate 1 coated with an opticalcoating 2. FIG. 1B shows that to protect the optical coating fromscratches and environmental corrosion during shipping and handling, acarbon protective layer 3 is deposited on the optical coating 2. FIG. 1Cshows that after tempering the optical coating 2 and/or the glasssubstrate 1 at elevated temperatures in an atmosphere reactive tocarbon, the carbon protective layer 3 is converted into a carboncontaining gas, eliminating any scratches or other defects that had beenpresent in the carbon protective layer 3.

The carbon protective layer is a layer consisting essentially of carbon.The term “consisting essentially of”, while not excluding unavoidableimpurities, excludes other unspecified elements and compounds that wouldbe left behind as a solid residue when the carbon is reacted tocompletion with a reactive atmosphere to form a carbon containing gas.In embodiments, the carbon layer consists of carbon and unavoidableimpurities.

The carbon layer can be deposited on the optical coating by a vapordeposition process. Techniques and processes for vapor depositing carbonare well known in the art. Suitable vapor deposition processes includeevaporation and plasma deposition processes such as plasma chemicalvapor deposition, ion implantation and sputtering. The sputtering can beDC or RF. An inert gas such as Ar, with or without small amounts ofadditional gases such as hydrogen and nitrogen, can be used in theplasma deposition processes to form the carbon layer. The presence of 1to 10% nitrogen in the inert gas favors the deposition of graphiticcarbon. The nitrogen in the inert gas can be used to dope the carbonwith nitrogen.

The carbon layer can include one or more phases of carbon, such asgraphite, diamond and amorphous phases of carbon. The carbon layer canalso include diamond-like carbon. The carbon in graphite has sp2bonding. The carbon in diamond has sp3 bonding. Amorphous carbongenerally includes both sp2 and sp3 bonding, but has no long rangeorder. Diamond-like carbon also includes both sp2 and sp3 bonding, andexhibits a hardness resembling that of diamond.

The carbon layer can be from 1 to 10 nm thick. A carbon layer less than1 nm thick does not provide adequate scratch resistance. A carbon layermore than 10 nm thick becomes difficult to remove completely in aatmosphere reactive to carbon.

The reactive atmosphere used to convert the carbon protective layer intoa carbon containing gas can include various gases known in the art to bereactive with carbon. For example, the reactive atmosphere can includehydrogen, which can convert the carbon into methane gas. A halogen, suchas fluorine or chlorine, can be used to form in at elevated temperaturesa tetrahalomethane gas such as CF₄ or CCl₄. Oxygen in a reactiveatmosphere can be used to form carbon monoxide and carbon dioxide gases.Because optical coatings and glasses generally contain various oxidesthat are inert in oxygen, the atmosphere reactive with carbon preferablycontains oxygen. Because air, which contains O₂, is inexpensive andreadily available, more preferably the reactive atmosphere is air.

Tempering is a process which involves heating a material to elevatedtemperatures and then quenching. Tempering is known to significantlyincrease the strength and toughness of glass and of optical coatings onglass. Glass can be tempered by heating to a temperature in the range of400 to 650° C. followed by quenching to room temperature. Opticalcoatings including Ag layers can be tempered by heating to a temperaturein a range below the 960° C. melting point of Ag followed by quenchingto room temperature. For example, a low-emissivity optical coatingincluding an Ag layer can be tempered by heating to about 730° C. for afew minutes following by quenching. Preferably, the glass and opticalcoatings are tempered at a temperature of at least 400° C. Inembodiments of the invention, both the glass and the optical coating aretempered in an oven held at an elevated temperature. In otherembodiments, to avoid having to heat the entire mass of the glass, onlythe optical coating is tempered. For example, instead of being heated inan oven, the optical coating can be heated by a flame or high intensitylamp to a temperature sufficient to both temper the optical coating andburn away the protective carbon layer.

Thus, tempering an optical coating covered with a carbon protectivelayer in an atmosphere reactive with carbon can cause the carbon to forma carbon containing gas and leave the surface of the optical coating.Any scratches in the carbon layer disappear along with the carbon layer.Preferably, the reactive atmosphere tempering removes all of the carbonprotective layer from the optical coating.

The carbon protective layer can protect an optical coating fromscratches caused during the manufacture of the coating by, e.g.,shipping and handling. In addition, the carbon protective layer canprotect an optical coating from corrosive environments that mightdevelop when the optical coating with the carbon protective layer isstored in air for one or more days or is washed. Preferably, the numberof scratches in the optical coating immediately after the carbonprotective layer is removed is no more than 110% of the number ofscratches in the optical coating immediately before the carbon wasdeposited on the optical coating.

In embodiments of the present invention, between the carbon protectivelayer and the optical coating, an SPB layer can be formed. Preferably,the SPB layer has a uniform composition and is homogeneous throughout.An SPB layer is made from a material having the property of inhibitingduring tempering the propagation of scratches and cracks into theoutermost layer of an optical coating. Different outermost layersrequire different materials in an SPB layer. The material forming theSPB layer should be less brittle and glass-like than the outermost layerof the optical coating. Preferably, the fracture toughness of the SPBmaterial is higher than that of the outermost layer.

FIG. 2 shows embodiments of the invention in which a SPB layer 4 issandwiched between a carbon protective layer 3 and an outermost Si₃N₄layer 2 a of an optical coating 2. Both the SPB layer 4 and the carbonprotective layer 3 provide scratch protection to the optical coating 2.In particular, the SPB layer 4 inhibits the propagation of scratches inthe carbon protective layer 3 down to and into the Si₃N₄ layer 2 a.

Preferably the silicon nitride outermost layer has a uniform compositionand is homogeneous throughout.

An outermost layer of amorphous silicon nitride (e.g., amorphous Si₃N₄)is preferred in an optical coating on glass subject to tempering.Amorphous silicon nitride does not undergo a phase change upon heatingto the temperatures necessary to temper glass. Furthermore, the densityof amorphous silicon nitride is the same before and after the tempering,so the tempering does not leave stresses at the interface of the siliconnitride and the rest of the optical coating that could lead todelamination.

The amorphous silicon nitride also inhibits the formation of haze in theoptical coating. Haze develops when materials mix together to form a twophase system causing the index of refraction to vary as a function ofposition throughout a layer. Because the phase stability of siliconnitride prevents mixing, the haze in optical coatings with an outermostsilicon nitride is low after tempering.

Since the silicon nitride remains amorphous, there is less atomic motionat the interfaces between layers of the optical coating than there wouldbe if there were a phase change, which results in better retention ofthe initial adhesion between layers.

A problem with an outermost layer of amorphous silicon nitride in anoptical coating is that the covalent bonding and amorphous structure ofthe silicon nitride results in a stiff material with crack propagationproperties similar to those of glass. Small cracks propagate easilythrough stiff, glassy materials.

FIG. 3 illustrates a possible mechanism by which cracks can propagatethrough an optical coating 2 having an outermost layer of siliconnitride. Initially small scratches are shallow and not detectable by the“naked eye” inspection methods used on most tempering lines. This isbecause the scratches do not penetrate completely through the outermostsilicon nitride. However, upon heating the small cracks propagatethrough the silicon nitride to underlying layers of, e.g., Ag. Onceexposed by the crack, the Ag can agglomerate at its unconstrainedsurface. When the Ag agglomerates, the crack becomes visible and thepart must be rejected.

In the embodiments shown in FIG. 2, cracks in tempered optical coatingswith silicon nitride outermost layers are minimized by depositing beforetempering an SPB layer on the silicon nitride and a C layer on the SPBlayer. The same sputtering equipment can be used to deposit the SPB/Ccombination and the optical coating onto glass.

As discussed above, carbon provides a classic low-friction surface. Evenwhen an abrasive initially scratches carbon, the abrasive becomes coatedwith carbon, leading to carbon-on-carbon sliding with extremely lowfriction.

If an abrasive succeeds in puncturing the protective carbon layer, thenthe abrasive will encounter the SPB layer. However, most scratches orcracks formed by the abrasive will not propagate through the SPB layerupon tempering. Although, unlike the carbon protective layer, the SPBlayer remains after tempering, most scratches in the SPB remaininvisible to the naked eye.

Suitable materials for forming an SPB layer include metals such as Ti,Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W; oxides of thesemetals; and nitrides of these metals.

The term “oxides” as used herein includes stoichiometric oxides;superoxides, containing more than a stoichiometric amount of oxygen; andsuboxides, containing less than a stoichiometric amount of oxygen. Theterm “metal suboxide” as used herein includes metals doped with smallamounts, e.g, 0.1-10 atomic %, of oxygen.

The term “nitrides” as used herein includes stoichiometric nitrides;supernitrides, containing more than a stoichiometric amount of nitrogen;and subnitrides, containing, less than a stoichiometric amount ofnitrogen. The term “metal subnitride” as used herein includes metalsdoped with small amounts, e.g., 0.1-10 atomic %, of nitrogen.

Suitable stoichiometric oxides for forming an SPB layer include TiO₂,SiO₂, ZnO, SnO₂, In₂O₃, ZrO₂, Al₂O₃, Cr₂O₃, Nb₂O₅, MoO₃, HfO₂, Ta₂O₅,WO₃. Suitable stoichiometric nitrides for forming an SPB layer includeTiN. TiO₂ in particular is very good at inhibiting scratches. The SPBlayer can be formed by vapor deposition techniques known in the art.

The SPB layer can be from 2 to 8 nm thick. When the SPB layer is astoichiometric oxide or nitride, the SPB layer is preferably from 2 to 8nm, more preferably from 3 to 6 nm, thick. When the SPB layer is ametal, the SPB layer is preferably from 4 to 8 nm, more preferably from4 to 6 nm, thick. If the stoichiometric oxide or nitride SPB layer isthinner than 2 nm, or the metal SPB layer is thinner than 4 nm, then theSPB material exhibits a decreased tendency to inhibit the propagation ofscratches. There is little advantage to an SPB layer thickness ofgreater than 8 nm, because the scratch propagation inhibition resultingfrom the SPB layer saturates at a thickness of about 8 nm and theinfluence of the SPB layer on the optical characteristics of an opticalcoating, which must be taken into account, increases with SPB layerthickness. However, as discussed below, metals, metal suboxides andmetal subnitrides can be used as diffusion barrier layers greater than 2nm thick that, after being oxidized during tempering, can form metaloxide SPB layers that can be substantially invisible.

As discussed above, in embodiments the SPB layer can be combined with acarbon protective layer on top of the SPB layer. In other embodiments,the SPB layer can form the only protective layer on an optical coating.An SPB layer can help to prevent scratching and scratch propagation onhandling, even without a protective carbon layer.

In embodiments of the invention, the SPB layer can be formed byoxidizing a diffusion barrier layer used to provide corrosion protectionto an optical coating before tempering. The diffusion barrier layer is ametal, metal suboxide or metal subnitride material including an metalelement selected from Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta andW. The diffusion barrier layer is deposited on the outermost layer of anoptical coating before tempering the optical coating. Tempering theoptical coating in an atmosphere containing oxygen converts thediffusion barrier layer into a metal oxide SPB layer. Preferably, thediffusion barrier layer contains Ti, Zr or Al, which upon heating in aircan be converted to SPB layers of the metal oxides TiO₂, ZrO₂ or Al₂O₃,respectively. Preferably, the metal suboxide contains about 80% or lessof the oxygen present in the most fully oxidized stoichiometric oxide ofthe metal. Metal suboxide films deposited with about 80% or less of fulloxidation tend to form better diffusion barriers than films reactivelydeposited with more than about 80% of full oxidation.

As discussed above, metal layers are known to be particularly effectivebarriers to diffusive movement of corrosive fluids. Metals suboxides andmetal subnitrides function similarly to metals as diffusion barriers.Metal suboxides and metal subnitrides tend to form dense layers whensputtered or evaporated and chemically inhibit diffusion of oxygen andwater vapor to a greater extent than the corresponding fully oxidizedmetals.

Metal suboxides and metal subnitrides can be formed by vapor depositionmethods known in the art. For example, metal suboxides and metalsubnitrides can be formed by vapor depositing a metal in an atmospherecontaining a controlled amount of oxygen and nitrogen.

Metal suboxides and subnitrides tend to be optically absorbing andreduce visible transmission of an optical coating until they are heatedand reacted to a fully oxidized state.

The bonding between nitrogen and metal in a metal subnitride istypically not as strong as the bonding between oxygen and metal in ametal suboxide. Heating a metal subnitride in an atmosphere containingoxygen will generally convert the metal subnitride to the correspondingmetal oxide or at least to a metal oxynitride that is substantiallytransparent.

The diffusion barrier layer can be between about 4 and 8 nm thick,preferably between 4 and 6 nm thick. Typically, reactive metal layerswill fully oxidize in room temperature air if the metal is 2 nm or lessin thickness. Thicker metal layers will often oxidize to a depth of 2 nmwhile the remainder of the layer remains metallic. The oxidation processcan be driven deeper if the metal is exposed to an energy source such asheat or a more chemically reactive environment than air. In embodimentsof the invention, the diffusion barrier layer is deposited thicker thanthe thickness which allows complete oxidation in room temperature air.In this way, the layer remains metallic and functions as an effectivecorrosion barrier prior to tempering. To provide the scratch propagationresistance discussed above before oxidation, preferably the diffusionbarrier layer is deposited to a thickness of 4 nm or more. To ensurethat the diffusion barrier layer is fully oxidized during the temperingprocess, the diffusion barrier layer is deposited to a thickness of 8 nmor less, preferably 6 nm or less.

When a metal, metal suboxide or metal subnitride layer 4 to 6 nm thickis fully oxidized, it tends to have little optical effect on the opticalstack. Because metal oxides are more transparent to visible light thanmetals, metal suboxides and metal subnitrides, fully oxidizing thediffusion barrier layer results in a metal oxide SPB layer that iseffectively optically invisible.

Using the tempering process to form a metal oxide SPB layer from adiffusion barrier on a temperable low-emissivity optical coating bothprotects the coating from corrosion before tempering and eliminates manyundesirable optical effects associated with having a diffusion barrierlayer as the SPB layer on the low-emissivity optical coating aftertempering.

In further embodiments, a carbon layer can be deposited onto thediffusion barrier layer on the temperable low-emissivity optical coatingas additional protection for the optical coating. Tempering the opticalcoatings by heating in air can then both burn away the carbon layer andconvert the diffusion barrier layer into a transparent metal oxide SPBlayer.

FIGS. 4A-4C illustrate embodiments of the invention in which a metaloxide SPB layer is formed by depositing a metal layer onto an opticalcoating and then reacting the metal in an atmosphere containing oxygento form the oxide. FIG. 4A shows a glass substrate 1 provided with anoptical coating 2. FIG. 4B shows a metal layer 5 deposited on theoptical coating 2. FIG. 4C shows that upon heating the metal layer 5 inan atmosphere containing oxygen, such as air, the metal layer 5 isconverted to a metal oxide scratch propagation blocker layer 4.

EXAMPLES

The following examples are intended to illustrate the invention furtherbut not to limit the field of use as defined in the attached patentclaims.

Example 1

FIGS. 5(1)-5(4) are optical microscope photographs showing thesignificant decrease in scratches that results according to the presentinvention by depositing a temporary carbon protective layer on anoptical coating before tempering, and then removing the carbonprotective layer by tempering in a reactive atmosphere. Each sample hadthe same optical coating. The optical coating included multiple layersof Zn, Ag, and NiCr, along with an outermost layer of 36 nm thick Si. Acarbon protective layer 1 nm thick was deposited on the optical coatingsof the samples shown in FIGS. 5(1) and 5(2), but not on the opticalcoatings of the samples shown in FIGS. 5(3) and 5(4). The samples werethen scratched under the same conditions using the same commercialabrasion wheel (a TABER® wheel). FIGS. 5(1) and 5(2) show differentareas of carbon protected samples representative of the worstscratching. The scratch in FIG. 5(1) is about 10-15 nm wide. FIGS. 5(1)and 5(3) show scratched samples before tempering. FIGS. 5(2) and 5(4)show scratched samples after tempering in air at 730° C. for fourminutes. During the tempering in air, the width of the scratches roughlydoubled. The carbon protective layer on the sample shown in FIG. 5(2)burned away during the tempering along with most of the scratches.

FIG. 5 shows that the presence of a carbon protective layer on anoptical coating before tempering greatly reduces the number of scratchesappearing on the optical coatings after tempering in air when the carbonlayer has burned away.

Example 2

FIG. 6 shows nine samples (numbered 1 through 9) comparing the effect ofdifferent carbon protective layer thicknesses on scratches remaining onoptical coatings after tempering. Each sample had the same opticalcoating. The optical coating included multiple layers of Zn, Ag, andNiCr, along with an outermost layer of 36 nm thick Si. Carbon protectivelayers of various thicknesses were deposited on the samples as shown inthe following Table 1. Samples 1-2 contained no carbon protective layer.

TABLE 1 SAMPLE CARBON THICKNESS (nm) 1 none 2 none 3 1 4 1.2 5 1.8 6 5 75 8 10 9 15

The samples were scratched under the same conditions using the samecommercial abrasion wheel (a TER® wheel). The nine samples were eachtempered in air at 730° C. for four minutes. FIG. 6 shows Samples 1-9after the tempering.

As shown in FIG. 6, Samples 3-9, which included temporary carbonprotective layers, had significantly fewer scratches after tempering inair than did Samples 1-2, which did not include carbon protectivelayers. The color of Samples 3-8 after tempering was the same as thecolor of Samples 1-2 before tempering, indicating that the carbon layeron Samples 3-8 was completely removed. A trace of carbon remained onSample 9 after the tempering.

Example 3

Individual protective layers of various SPB materials and carbon weredeposited onto identical optical coatings on glass. The protectivelayers were scratched under the same conditions using the samecommercial abrasion wheel (a TABER® wheel). Table 2 shows the relativeabilities of individual SPB materials and of carbon to lessen scratchdamage.

TABLE 2 PROTECTIVE LAYER (SPB or C) THICKNESS (nm) DAMAGE (%)unprotected (standard) — 100 SiO₂ 2 60 TiN 2 30 TiO₂ 2 30 ZnO 2 10 C 110 C 10  2

In Table 1, the “% Damage” is the approximate number of scratches perunit length perpendicular to the direction of the abrasive tool.

Table 2 shows that an SPB layer can help to prevent scratching andscratch propagation on handling, even without a protective carbon layer.Combined, the SPB and C layers can have an even greater effect ininhibiting scratches. The thicknesses of each SPB and C layer can bevaried as needed.

Example 4

Zr layers of different thicknesses are deposited onto identical silverbased low-emissivity optical coatings on glass substrates. The Zr coatedoptical coatings are exposed to room temperature air having a relativehumidity of 80% for 24 hours. The optical coatings are then tempered at730° C. in air. Zr layers 2 nm and 3 nm thick are found to provide nocorrosion protection to the silver based low-emissivity coatings. Incontrast, Zr layers 4 nm and 8 nm thick are found to provide substantialcorrosion protection to the silver based low-emissivity coatings.

While the present invention has been described with respect to specificembodiments, it is not confined to the specific details set forth, butincludes various changes and modifications that may suggest themselvesto those skilled in the art, all falling within the scope of theinvention as defined by the following claims.

1. A transparent article comprising a transparent substrate; an opticalcoating comprising one or more layers on the substrate, where the one ormore layers include furthest from the substrate a homogeneous outermostlayer comprising amorphous silicon nitride; and a protective coating onthe outermost layer, wherein the protective coating consists of ascratch propagation blocker layer on the outermost layer, and a layerconsisting essentially of carbon on the scratch propagation blockerlayer; and the scratch propagation blocker layer is a homogeneous layercomprising a material selected from the group consisting of Ti, Zr, Cr,Nb, Hf, and Ta; oxides of Ti, Zr, Cr, Nb, Hf, and Ta; nitrides of Ti,Zr, Cr, Nb, Hf, and Ta; oxynitrides of Ti, Zr, Cr, Nb, Hf, and Ta; andmixtures thereof.
 2. The transparent article according to claim 1,wherein the substrate comprises a glass.
 3. The transparent articleaccording to claim 2, wherein the substrate comprises a tempered glass.4. The transparent article according to claim 1, wherein the opticalcoating is a low-emissivity coating.
 5. The transparent articleaccording to claim 1, wherein the optical coating is a tempered coating.6. The transparent article according to claim 1, wherein the opticalcoating contains at least one layer comprising Ag.
 7. The transparentarticle according to claim 1, wherein the fracture toughness of thescratch propagation blocker layer is higher than the fracture toughnessof the outermost layer.
 8. The transparent article according to claim 1,wherein the scratch propagation blocker layer is from 2 to 8 nm thick.9. The transparent article according to claim 8, wherein the scratchpropagation blocker layer comprises a material selected from the groupconsisting of oxides of Ti, Zr, Cr, Nb, Hf, and Ta.
 10. The transparentarticle according to claim 8, wherein the scratch propagation blockerlayer comprises a material selected from the group consisting ofoxynitrides of Ti, Zr, Cr, Nb, Hf, and Ta.
 11. The transparent articleaccording to claim 1, wherein the layer consisting essentially of carbonis doped with nitrogen.
 12. The transparent article according to claim1, wherein the layer consisting essentially of carbon consists of carbonand unavoidable impurities.
 13. The transparent article according toclaim 1, wherein the carbon in the layer consisting essentially ofcarbon comprises at least one form of carbon selected from the groupconsisting of diamond-like carbon and graphite.
 14. The transparentarticle according to claim 1, wherein the layer consisting essentiallyof carbon is from 1 to 10 nm thick.
 15. A transparent article comprisinga transparent substrate; an optical coating comprising one or morelayers on the substrate, where the one or more layers include furthestfrom the substrate a homogeneous outermost layer comprising siliconnitride; and a protective coating on the outermost layer, wherein theprotective coating consists of a scratch propagation blocker layer onthe outermost layer, and a layer consisting essentially of carbon on thescratch propagation blocker layer; and the scratch propagation blockerlayer is a homogeneous layer 2 to 8 nm thick comprising a materialselected from the group consisting of Ti, Zr, Cr, Nb, Hf, and Ta; oxidesof Ti, Zr, Cr, Nb, Hf, and Ta; nitrides of Ti, Zr, Cr, Nb, Hf, and Ta;oxynitrides of Ti, Zr, Cr, Nb, Hf, and Ta; and mixtures thereof.
 16. Thetransparent article according to claim 15, wherein the substratecomprises a glass.
 17. The transparent article according to claim 16,wherein the substrate comprises a tempered glass.
 18. The transparentarticle according to claim 15, wherein the optical coating is alow-emissivity coating.
 19. The transparent article according to claim15, wherein the optical coating is a tempered coating.
 20. Thetransparent article according to claim 15, wherein the optical coatingcontains at least one layer comprising Ag.
 21. The transparent articleaccording to claim 15, wherein the fracture toughness of the scratchpropagation blocker layer is higher than the fracture toughness of theoutermost layer.
 22. The transparent article according to claim 15,wherein the scratch propagation blocker layer comprises a materialselected from the group consisting of oxides of Ti, Zr, Cr, Nb, Hf, andTa.
 23. The transparent article according to claim 15, wherein thescratch propagation blocker layer comprises a material selected from thegroup consisting of oxynitrides of Ti, Zr, Cr, Nb, Hf, and Ta.
 24. Thetransparent article according to claim 15, wherein the layer consistingessentially of carbon is doped with nitrogen.
 25. The transparentarticle according to claim 15, wherein the layer consisting essentiallyof carbon consists of carbon and unavoidable impurities.
 26. Thetransparent article according to claim 15, wherein the carbon in thelayer consisting essentially of carbon comprises at least one form ofcarbon selected from the group consisting of diamond-like carbon andgraphite.
 27. The transparent article according to claim 15, wherein thelayer consisting essentially of carbon is from 1 to 10 nm thick.
 28. Atransparent article comprising a transparent substrate; an opticalcoating comprising one or more layers on the substrate, where the one ormore layers include furthest from the substrate a homogeneous outermostlayer comprising silicon nitride; and a protective coating on theoutermost layer, wherein the protective coating consists of a scratchpropagation blocker layer on the outermost layer; and the scratchpropagation blocker layer is a homogeneous layer comprising a materialselected from the group consisting of Ti, Zr, Cr, Nb, Hf, and Ta; oxidesof Ti, Zr, Cr, Nb, Hf, and Ta; nitrides of Ti, Zr, Cr, Nb, Hf, and Ta;oxynitrides of Ti, Zr, Cr, Nb, Hf, and Ta; and mixtures thereof.
 29. Thetransparent article according to claim 28, wherein the substratecomprises a glass.
 30. The transparent article according to claim 29,wherein the substrate comprises a tempered glass.
 31. The transparentarticle according to claim 28, wherein the optical coating is alow-emissivity coating.
 32. The transparent article according to claim28, wherein the optical coating is a tempered coating.
 33. Thetransparent article according to claim 28, wherein the optical coatingcontains at least one layer comprising Ag.
 34. The transparent articleaccording to claim 28, wherein the outermost layer comprises amorphoussilicon nitride.
 35. The transparent article according to claim 28,wherein the fracture toughness of the scratch propagation blocker layeris higher than the fracture toughness of the outermost layer.
 36. Thetransparent article according to claim 28, wherein the scratchpropagation blocker layer is from 2 to 8 nm thick.
 37. The transparentarticle according to claim 36, wherein the scratch propagation blockerlayer comprises a material selected from the group consisting of oxidesof Ti, Zr, Cr, Nb, Hf, and Ta.
 38. The transparent article according toclaim 36, wherein the scratch propagation blocker layer comprises amaterial selected from the group consisting of oxynitrides of Ti, Zr,Cr, Nb, Hf, and Ta.
 39. The transparent article according to claim 28,wherein the scratch propagation blocker layer is in contact with air.40. The transparent article according to claim 1, wherein the scratchpropagation blocker layer is from 2 to 8 nm thick; and the layerconsisting essentially of carbon is from 1 to 10 nm thick.
 41. Thetransparent article according to claim 1, wherein the scratchpropagation blocker layer comprises a material selected from the groupconsisting of oxides of Zr; nitrides of Zr; oxynitrides of Zr; andmixtures thereof.
 42. The transparent article according to claim 15,wherein the scratch propagation blocker layer comprises a materialselected from the group consisting of oxides of Zr; nitrides of Zr;oxynitrides of Zr; and mixtures thereof.
 43. The transparent articleaccording to claim 28, wherein the scratch propagation blocker layercomprises a material selected from the group consisting of oxides of Zr;nitrides of Zr; oxynitrides of Zr; and mixtures thereof.
 44. Thetransparent article according to claim 1, wherein the scratchpropagation blocker layer comprises a material selected from the groupconsisting of oxides of Hf; nitrides of Hf; oxynitrides of Hf; andmixtures thereof.
 45. The transparent article according to claim 15,wherein the scratch propagation blocker layer comprises a materialselected from the group consisting of oxides of Hf; nitrides of Hf;oxynitrides of Hf; and mixtures thereof.
 46. The transparent articleaccording to claim 28, wherein the scratch propagation blocker layercomprises a material selected from the group consisting of oxides of Hf;nitrides of Hf; oxynitrides of Hf; and mixtures thereof.
 47. Thetransparent article according to claim 1, wherein the scratchpropagation blocker layer comprises a material selected from the groupconsisting of oxides of Ti, Zr, Hf, Cr, and Ta; nitrides of Zr; andoxynitrides of Zr.
 48. The transparent article according to claim 15,wherein the scratch propagation blocker layer comprises a materialselected from the group consisting of oxides of Ti, Zr, Hf, Cr, and Ta;nitrides of Zr; and oxynitrides of Zr.
 49. The transparent articleaccording to claim 28, wherein the scratch propagation blocker layercomprises a material selected from the group consisting of oxides of Ti,Zr, Hf, Cr, and Ta; nitrides of Zr; and oxynitrides of Zr.